I have extensively toured France for many years and, unlike you, always found the autoroute service station prices noticeably higher than the supermarkets, which I normally use.
Perhaps not quite the difference you quoted, but I always avoided them. The in-town stations prices were mostly a little above the supermarkets.
Supermarket prices do vary, but mostly Carrefour was as good as any. My tank takes 60 litres, so one fill-up keeps me going for quite a while.
Finally, if you were recently only paying 1.26euros per litre for lead free 95, you were getting a real bargain!.
Looking today at the official French government site of fuel prices for the Burgundy region 1.31 euros/l was the cheapest I could see. This rose to 1.46 in some outlets.
The official French fuel price site is here: